1. Field of the Invention
This invention relates to an optical recording medium suitable for writing and reproduction of information with a laser beam, in particular, one emitted from a semiconductor laser. More particularly, this invention relates to an optical recording medium which has improved properties under harsh environmental conditions and can be used in optical discs, optical cards and the like.
2. Description of Prior Art
Generally speaking, an optical recording medium such as an optical disc or optical card can record information at high density by forming optically detectable small pits of, for example, about 1 .mu.m on a thin recording layer having spiral, circular, or straight line grooves provided on a substrate. When a converged laser beam is scanned along the grooves on the surface of the recording layer, the recording layer absorbs the laser energy at the spots irradiated to form optically detectable pits, whereby information is written.
For example, according to a heat mode recording tenchnique, the recording layer absorbs heat energy to form small concave pits by evaporation or melting at the sites that have absorbed the heat energy. According to an other heat mode recording technique, pits having optically detectable density difference are formed at the sites that have absorbed laser energy.
The information recorded in the optical recording medium is detected by scanning a laser beam along the grooves and reading the optical differences between the portions where the pits are formed and the portions where no pit is formed.
For instance, a laser is irradiated to scan the surface of the recording layer along the grooves and the laser energy reflected is monitored with a photodetector. When the pit portions are irradiated, the output of the photodetector is low; when the pit-free portions are irradiated, the laser is reflected sufficiently, thereby increasing the output of the photodetector.
For the above optical recording medium for use as an optical disc and optical card, so far there have been proposed a metallic thin film such as an aluminum evaporated film, and a thin film mainly consisted of an inorganic material such as bismuth, tellurium oxide or a chalcogenide-base amorphous glass. These thin films are generally sensitive to light rays of wavelength ranging from 350 to 800 nm and have a high reflectance for laser beams, and hence exhibit lower efficiency of laser energy utilization and poor sensitivity characteriatics.
In recent years, there have been developed semiconductor laser devices of small size and low cost. Further, laser emitted from these devices can be directly modulated. However, most of these lasers have a wavelength of at least 750 nm. Accordingly, in order to carry out recording and/or reproduction with such a long wavelength semiconductor laser, the recording layer should have an absorption band in a long wavelength region, generally 750-850 nm.
For those reasons, there has recently been researched an organic thin film which can develop optical changes in physical properties with light energy at relatively long wavelengths (e.g., greater than 780 nm). This type of organic film is effective in that pits can be formed on the film by a semiconductor laser having an oscillation wave-length of around 780 nm or 830 nm. Examples of such organic films are those containing squarylium or croconium-base dyes disclosed in U.S. Pat. No. 4,548,886.
In general, however, those organic compounds which have an absorption band in the longer wavelength side are unstable against heat and light.
More specifically, since optical recording media are not always handled in offices full-equipped with air conditioning systems, care should be taken in harsh conditions such as storage in transporting warehouses and the rising temperatures in a drive unit. A medium superior in stability at high temperatures is therefore desired. When the optical recording medium is used as an optical card, it is difficult to make the card into a hollow structure (i.e., air sandwiched structure) from considerations of thickness and strength. The optical card is, therefore, necessarily made into a laminated structure by bonding a pair of opposite substrates directly to a recording layer using an adhesive. For the adhesive used, a hotmelt adhesive is effective because it is superior in operability without degrading the writing sensitivity of the information on the recording layer. However, the hotmelt adhesive is exposed to a high temperature of about 100.degree. C., though for a short period of time when the substrates are bonded to the recording layer. Therefore, there is a need for a recording layer that is more resistant against heat.
Further, optical cards are personally carried and handled in most cases, and highly liable to be treated in a rough manner. There is, therefore, also a need for an optical card having excellent stability under harsh environmental conditions besides heat-resistance.
In addition, the type of optical recording medium that uses an organic compound mentioned above to form the recording layer accompanies the problem that, over aging, the organic compound tends to deteriorate upon repeated irradiation of regenerating (reproduction) light or natural light. There is, therefore, a need for an optical recording medium also having excellent stability against light.